Justin Smith will turn 34 in September and will be coming off a torn triceps.

Photo: Marcio Jose Sanchez, Associated Press

Justin Smith will turn 34 in September and will be coming off a...

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Alabama defensive lineman Jesse Williams (54) arrives for Media Day on Friday, Jan 6, 2012 at the Superdome in New Orleans. Alabama will face LSU in the BCS Championship game at the Superdome on Monday. (AP Photo/Dave Martin)

Senior Bowl South Squad defensive lineman John Jenkins of Georgia (6) talks with running back Mike Gillislee of Florida in the first half of the Senior Bowl college football game at Ladd-Peebles Stadium in Mobile, Ala., Saturday, Jan. 26, 2013. (AP Photo/Dave Martin)

Photo: Dave Martin, Associated Press

Senior Bowl South Squad defensive lineman John Jenkins of Georgia...

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The 49ers reportedly have a deal in place to trade Alex Smith, but it cannot be executed until March 12.

Justin Smith's full value to the 49ers was best displayed after his triceps tendon was half torn.

The Niners would have identified their defensive line as an area they needed to address in the draft, but their performance after Smith's mid-December injury surely heightened the sense of urgency.

What would life be like with an absent or diminished Smith, who will turn 34 in September and is entering the last year of his contract?

The 49ers got a harrowing peek into the future when the All-Pro defensive tackle was injured early in the third quarter of a 41-34 win at New England, missed the final two regular-season games and returned for three postseason games sporting a bulky brace on his left arm. Before the Super Bowl, Smith said his triceps was at least 50 percent torn.

In their first 13 1/2 games, the 49ers' defense allowed an average of about three points per quarter. After Smith was injured, they allowed about seven. Similarly, opposing quarterbacks had a 75.3 rating before Smith's injury. That figure ballooned to 100.6 in the final 22 quarters after he was sidelined.

Finally, outside linebacker Aldon Smith didn't have a sack in the final six games after posting 19 1/2 in the first 13 regular-season games while working in tandem with Smith.

Beyond the need to find Smith's heir apparent, there are other reasons to address the defensive line in the draft.

The lack of depth at the position explains the heavy workload of Smith and defensive tackle Ray McDonald in recent years. Including playoff games, McDonald ranks third in snaps played by defensive linemen since 2011 and Smith is fourth, according to ESPN.

The 49ers haven't selected a true defensive lineman in the first six rounds since drafting noted bust Kentwan Balmer in the first round in 2008. That streak will end this season, and it's a good year for the 49ers to shop: The draft is loaded with defensive linemen, and San Francisco has four of the first 93 picks over the first three rounds.

The defensive linemen and linebackers will perform on-field drills Monday at the NFL combine in Indianapolis. Georgia's John Jenkins and Alabama's Jesse Williams are two players to watch.

Jenkins and Williams both played nose guard and tackle in the 3-4 scheme employed by the 49ers and excelled against top-level competition in the SEC. They could be available when the 49ers draft with the 31st pick overall.

Defensive line draft possibilities

JESSE WILLIAMS, ALABAMA

Height/weight: 6-3, 323

Skinny: Strength isn't an issue for Williams, who bench-pressed 600 pounds during college. Projected as a nose tackle in the NFL, he hopes to use the combine to answer questions about his movement skills. The native of Australia grew up playing rugby and didn't try football until he was 14. At Alabama, Williams played defensive end, nose guard and tackle on passing downs in four-man fronts. As a senior, he played nose guard and anchored the nation's top-ranked run defense.

Quote: "He takes on blocks really well. He's just kind of a get-dirty player. He does all the little things you ask of him. He's very consistent and he keeps getting better every year." - ESPN's Todd McShay

JOHN JENKINS, GEORGIA

Height/weight: 6-4, 346

Skinny: Jenkins has dropped 24 pounds since playing his senior season at 370. He played both nose guard and defensive tackle in college and hopes to prove he could have similar versatility in the NFL. His college career ended prematurely this past season when he lost academic eligibility before Georgia's bowl game. At the combine, he said he ballooned to 370 pounds because he was studying: "Instead of me doing extra running, I was doing extra tutoring."

Quote: He's not as consistent as (Jesse Williams), but he's a more explosive player." - McShay

Skinny: Hankins dropped about 30 pounds during college and left Ohio State a year early after the NFL draft advisory board told him he was a likely first-round pick. Hankins is viewed as ultra-talented, but there are questions about his lack of consistency. He followed a breakout sophomore season with 12 fewer tackles, seven fewer tackles for losses and two fewer sacks in 2012.

Quote: "He's probably more talented than Williams or Jenkins, but he's got kind of a bad body. He's got too much weight around the middle and he's played very inconsistently through his career. So he's a guy that has the talent to go much higher, but could fall down the board a little bit. He has talent that absolutely justifies gambling on him." - CBS Sports' Rob Rang

- Eric Branch

Smith trade reportedly set but awaiting start of league year

The 49ers can't finalize a trade involving quarterback Alex Smith until March 12, but they reportedly feel confident about crossing off that item from their to-do list for the offseason.

According to CBS Sports, the 49ers have told other teams that a deal is "effectively complete." It is not known to whom the 49ers would trade Smith, or what they would net in return. It also is not known whether the Niners are circulating a-deal-is-all-but-complete talk to stimulate interest in the trade market. Finally, the 49ers or the mystery team could back out of a handshake deal at any time with the start of the league year more than two weeks away.

Two logical trade partners, the Jaguars and Browns, are not involved, according to reports from the Florida Times-Union and Cleveland Plain Dealer.

Kansas City is viewed as another logical landing spot for Smith. Before the Super Bowl, ESPN analyst Trent Dilfer, a former teammate and a close friend of Smith's, told The Chronicle that new Chiefs head coach Andy Reid "really likes Alex." According to Dilfer, Reid, then with the Eagles, courted Smith when he was a free agent in 2012.

The Chiefs went 2-14 last season. Matt Cassel and Brady Quinn, a pending free agent, each started eight games. The duo had eight touchdowns and 20 interceptions and posted a 63.9 passer rating. Kansas City is not expected to use the No. 1 overall pick on a quarterback.